This invention relates to the provision of an hermetic design of optical fibre feed-through that is capable of being used for feed-through of polarisation maintaining fibre. Hermetic feed-through suitable for the feed-through of circularly symmetric optical fibre are known, and have been in use for some years. Thus in the laser of the laser package described in GB 2 124 402A the package is provided with an optical fibre pigtail that is hermetically sealed through one wall of the package. This seal is provided by metallising the fibre over a portion of its length, soldering with gold tin solder that metallised region within the bore of a length of support tube and then soft soldering the end of the support tube within the bore of a feed-through tube that has previously been sealed by brazing in an aperture extending through the package wall.
Typically the support tube is constituted by a length of hypodermic tubing, and soft solder can be substituted for the gold-tin solder employed for securing the metallised optical fibre within the bore of the hypodermic tubing. Though this sealing method is satisfactory for feed-through of circularly symmetric optical fibre, it has been found quite unsatisfactory for feed-through of polarisation maintaining fibre because the sealing process is found to disrupt the polarisation maintaining properties of the polarisation maintaining fibre. An explanation for this postulates that soldering the fibre within the hypodermic tube sets up a non-uniform stress field within the fibre that disrupts the stress field previously present in the fibre to provide its polarisation maintaining properties.